The present disclosure relates generally to switched mode power supplies, more particularly to the switched mode power supplies using current-controllable devices as power switches.
Switched mode power supplies commonly utilize power switches to control the current flowing through inductive devices. In comparison with other kinds of power supply, switched mode power supplies usually enjoy compact size and excellent conversion efficiency, and are accordingly welcome in the industry of power supplies
Bipolar junction transistor (BJT), a kind of power switch, excels in simple device structure, cheap price, and low conduction loss, such that it is well adopted in low cost applications. Unlike a metal-oxide-semiconductor transistor (MOS), which is another kind of power switch and is driven according to its gate voltage, a BJT is a current-controllable device, requiring a current control apparatus to control the base current Ib flowing through the base electrode of the BJT. Base current Ib and collector current Ic merge together to become emitter current Ie. The difference between emitter current Ie and collector current Ic could render, in a power supply, mismatch of output regulation or misjudgment for abnormal events. Furthermore, the switching speed of a BJT is known to be slower than that of a MOS, and it is desired in the art of circuit design of switched mode power supplies to quickly turn ON and OFF a BJT.
In this specification, the devices or apparatuses share the same reference characters have the same or similar function, structure, or characteristic, and can be obviously derived by a person skilled in the art based on the teaching herein. It is not required that they are exactly identical, however, and some might not be redundantly explained in consideration of brevity.